Viral vectors can provide an effective delivery system for transferring genetic material into a cell. Nucleic acids within many viral vectors, such as gamma-retroviral and lentiviral vectors, can integrate into the genome of a host cell. Cis-acting elements, such as viral post-transcriptional regulatory elements (PREs), including hepatitis virus PREs and variants thereof, have been used in viral vectors to enhance gene expression. Nucleic acids, expression cassettes, and vectors containing such elements are available. There is a need for improved PRE-containing nucleic acids, cassettes and vectors. Provided are embodiments that address such needs.